Commercial paint spraying operations are typically performed in spray painting booths. During these operations, less than half of the paint used may actually coat the surface to be painted, with the remainder representing overspray. In the spray application of polyurethanes, epoxy resins, lacquers, enamels, and additional coatings, it is normally necessary to trap the oversprayed coating materials in some way to avoid their building up on the walls and exhaust system surfaces of the spray booth. In many large industrial installations, including especially auto body painting systems, this is accomplished by the use of water curtains on the interior booth walls, whereby the oversprayed coating material is trapped in water which cascades down the walls of the booth and lies underneath it. Normally this water is recirculated over the walls from a reservoir of water underneath the booth or in close proximity to it by means of pumps which are capable of moving a large flow of water through large diameter pipes. However, the agglomeration and accumulation of live coating material in the water supply of these installations results in serious problems, such as blockage of the pipes and pumps which circulate the water, and a build-up of paint on the walls of the booth beyond the water curtain. As more and more coating material is sprayed in the booth, the material removed from the air builds up in the water in the form of a tar-like coherent sludge which in a short time can foul the pumps and lines which circulate the booth's water. Furthermore, this sludge is extremely difficult to remove from the pump, lines, reservoir, and other internal surfaces of the system with which it comes in contact. The accumulation of raw paint masses in the bottom of the water reservoir also creates a serious problem when the system is periodically cleaned out, much effort being required to remove the heavy build-up of coating material on the bottom of the reservoir.
It is, therefore, desirable to treat the water in the booth in such a way as to render the oversprayed coating material free of stickiness and tackiness so that it will readily separate itself from the water, not adhere to the curtain walls, pipes, pumps and other internals of the spray booth system, and remain as a detackified floating sludge.
One approach to detackification has been to combine certain polymeric materials with amphoteric metals. Thus, for example, U.S. Pat. No. 3,861,887 discloses treatment of paint booth wash water with a blend of polycationic water dispersible polymer with a water-soluble salt of an amphoteric metal to reduce the tackiness of paint. One problem with this approach is that use of metals, such as zinc, can create additional disposal concerns for the wastewater and sludge recovered from it.
Another approach has been to use clay-based treatment. For example, U.S. Pat. No. 4,504,395 discloses that certain hectorite clays can be effectively used to detackify oversprayed paint. A problem with this approach is that the sludge produced using clays can be difficult to dewater, resulting in larger volumes of potentially hazardous materials requiring haul-out to a secure landfill.
Other approaches using various treatments have also reportedly been considered. However, there remains a need for effective paint spray treatment which detackifies paint overspray and which does not aggrevate waste sludge disposal problems. Moreover, some paint spray facilities have found that predilution of paints with various organic solvent combinations, for example, methylethyl ketone, toluene, xylene, etc. can be practiced advantageously to improve the coating finish and economize upon paint wastage, and paint spray treatment which can effectively detackify overspray containing prediluted paint formulations can be especially valuable.